I/O Lib 15.1 should install without any 'special tricks' on 32-bit Vista. You should not need to set Compatiblity mode. What kind of problems do you run into? 64-bit Vista is another story...
I was told by an Agilent support engineer that a new version of the IO libs for 64-bit systems was close on the horizon, the best answer my be to wait a few weeks. I don't know of any other way to slip the libs into Vista 64, unless it's by running everything inside a virtualized OS within Vista.
64-bit Vista is another story...